1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission in an automobile. The invention specifically relates to a hydraulic control device for controlling oil pressures for predetermined frictional engagement elements and other frictional engagement elements in a clutch-to-clutch gearshift to engage the predetermined frictional engagement elements.
2. Background of the Invention
U.S. Pat. No. 5,368,531 discloses a method for performing hydraulic servo control for an automatic transmission where, at the time of an up-shift to a predetermined speed, for example a gear-change from second gear to third gear (wherein a brake B-2 is engaged and at the same time a brake B-3 is disengaged), a supply pressure (B-2 pressure) to a hydraulic servo and an accumulator on the engagement side acts on a control chamber of a 2-3 timing valve. A release pressure (B-3 pressure) communicating with a hydraulic servo and an accumulator on the disengagement side is decreased so that the engagement pressure and the release pressure are adjusted to be inversely proportional. The release pressure is controlled to decrease linearly from the point in time when the speed-change starts until at least the end of the torque phase.
The foregoing method for performing hydraulic servo control allows change of engagement and disengagement timings in accordance with change in the throttle opening during gear-shifting, unlike other methods where the pressure is released at a predetermined timing in one setting. As a result, gear-shift shock can be reduced.
However, in the above-mentioned method for performing hydraulic servo control, oil pressure supplied to the disengagement side hydraulic servo is released linearly with predetermined timing irrespective of the input torque and the temperature of the hydraulic fluid supplied to the hydraulic servo and, as a consequence, when the input torque and/or the oil temperature are high, engine racing can occur. Further, low input torque and/or low oil temperature can cause tie-up. Therefore, a need exists for development of a hydraulic control device that can decrease the disengagement side oil pressure in accordance with the magnitude of the input torque and the oil temperature.
Accordingly, it is an object of the present invention to provide a hydraulic control device that solves the above-described problem by controlling the disengagement side oil pressure in accordance with the magnitude of the input torque and the oil temperature.
To achieve the foregoing object, the present invention provides a hydraulic control device for an automatic transmission which includes an input shaft which receives torque from an output shaft of an engine, an output shaft connecting to drive wheels, a plurality of frictional engagement elements for changing a torque transfer path between the input and output shafts, and hydraulic servos for engaging or disengaging the frictional engagement elements. In the transmission, up-shift to a predetermined gear ratio is achieved by engaging first frictional engagement elements of the plurality of frictional engagement elements at a predetermined engagement side oil pressure and disengaging second frictional engagement elements at a predetermined disengagement side oil pressure. The hydraulic control device is provided with input torque calculation means for calculating input torque, and gear-change execution control means for controlling the oil pressure so as to change timing of draining of the disengagement side oil pressure acting on the second frictional engagements elements (xe2x80x9cdrain timingxe2x80x9d) in dependency on the magnitude of the input torque calculated by the input torque calculation means in torque phase control wherein both of the engagement side oil pressure and the disengagement side oil pressure are controlled.
In a preferred embodiment of the invention, the gear-change execution control means controls the drain timing (T1, T3) of the disengagement side oil pressure acting on the second frictional engagement elements in accordance with increase of oil temperature in the torque phase control wherein both the engagement side oil pressure and the disengagement side oil pressure are controlled.
The gear-change execution control means preferably controls a drain gradient (xcex4P) of the disengagement side oil pressure acting on the second frictional engagement elements in accordance with the magnitude of the input torque in the torque phase control wherein both the engagement side oil pressure and the disengagement side oil pressure are controlled.
In another preferred embodiment, the gear-change execution control means controls a drain gradient (xcex4P) of the oil pressure acting on the second frictional engagement elements in accordance with elevation of the oil temperature in the torque phase control wherein both the engagement side oil pressure and the disengagement side oil pressure are controlled.
The gear-change execution control means preferably advances the drain timing (T1, T3) as the input torque decreases. Likewise, the gear-change execution control means preferably advances the drain timing (T1, T3) as the oil temperature decreases. Further, the gear-change execution control means preferably makes the drain gradient (xcex4P) of the oil pressure steeper as the oil temperature increases.
Symbols appearing above in parentheses are used for the purpose of correlation with the drawings and are not intended to impose any limitation on the scope of the present invention.
Control of the drain timing (T1, T3) of the disengagement side oil pressure acting on the second frictional engagement elements in accordance with the magnitude of the input torque, in torque phase control wherein both of the engagement side oil pressure and the disengagement side oil pressure are controlled, makes it possible to prevent the tie-up and the engine racing that might otherwise result from the changing over of engagement.
The preferred control of the release time of the disengagement side oil pressure, taking both the oil temperature and the magnitude of the input torque into consideration, allows more effective control of the automatic transmission especially in cold regions.
Preferably, the drain gradient (xcex4P) of the disengagement side oil pressure acting on the second frictional engagement elements is controlled in accordance with the magnitude of the input torque. The release speed of the disengagement side frictional engagement elements is thus controlled in accordance with the input torque, making it possible to effectively prevent engine racing and the tie-up.
As noted above, the drain gradient (xcex4P) of oil pressure acting on the second frictional engagement elements is preferably controlled in accordance with elevation of the oil temperature. Such a delicate control becomes possible by, at high oil temperatures, making the drain gradient steeper while coping with the response delay at around zero oil pressure, and, at low oil temperatures, making the oil draining slower so that the draining is controlled in accordance with the state of the engagement side.
As also noted above, the drain timing (T1, T3) of the oil pressure is advanced in time as the input torque becomes smaller. This feature prevents tie-up due to drag torque at a low oil temperature. Moreover, at a high oil temperature and high input torque, control delaying the drain timing may prevent the engine racing due to the engagement delay in the engagement side frictional engagement element.
The preferred embodiment wherein the control advances the drain timing (T1, T3) of the oil pressure (to an earlier point in time) as the oil temperature decreases makes it possible to have a release operation account for the response delay in the disengagement side frictional engagement elements, thus preventing tie-up.
The preferred control wherein the drain gradient of the oil pressure is made steeper as the oil temperature increases makes it possible to drain the disengagement side at an advanced time to cope with the response delay at around zero oil pressure at a high oil temperature.